How can reseal the main drain in a concrete pool?

Though I've never worked on Pools themselves , I've pulled off small miracles using Hydraulic Cement inside concrete structures over the years. Many contractors use it for sealing watertight concrete structures such as oil separators , catch basins , and even Home Basements. Most versions of hydraulic cement will dry very fast , so be ready to go when you mix it up.

Make sure it is not a leak in the pipe by pressure testing it first. If it is leaking around the main drain pipe, that will be resealed if you are resurfacing the pool. If not you can use something like Sealboss 1510 which is injected into the crack around the pipe and it will expand 30-40 times the crack width forcing itself deep into the crack and has some flex to it. I have used it to seal up cracks many times including vanishing edge walls.